Those are very good questions:
It all depends on how the mechanism is used. It can be a compact mechanism, capable of reasonably high loads for a small CNC machine.
For heavier loads (>10kg? I don't have the means to measure forces accurately), proper bearings should be used.

Take it as a concept, for our final CNC mill we will probably be using a similar mechanism but with spinning threaded rod. It is as compact but less complex.